Medical equipment for radiation therapy treats tumorous tissue with high energy radiation. The dose and placement of the dose must be accurately controlled to ensure both that the tumor receives sufficient radiation to be destroyed, and that damage to the surrounding and adjacent non-tumorous tissue is minimized.
Internal source radiation therapy places capsules of radioactive material inside the patient in proximity to the tumorous tissue. Dose and placement are accurately controlled by the physical positioning of the isotope; however, internal source radiation therapy has the disadvantage of any surgically invasive procedure, including discomfort to the patient and risk of infection.
External source radiation therapy uses a radiation source that is external to the patient. The external source produces a collimated beam of radiation directed into the patient to the tumor site. External source radiation therapy avoids some of the problems of internal source radiation therapy, but it undesirably and necessarily irradiates a significant volume of non-tumorous or healthy tissue in the path of the radiation beam to the tumorous tissue.
The adverse effect of irradiating healthy tissue may be reduced while still maintaining a given dose of radiation in the tumorous tissue by projecting the external radiation beam into the patient at a variety of "gantry angles" with the beams converging on the tumor site. In this way, the particular volume elements of healthy tissue along the path of the radiation beam change as the gantry angle changes reducing the total dose to each such element of healthy tissue. The tumor, at the site of convergence of the beams, receives a cumulative, greater dose.
Radiation therapy systems employing high energy x-rays, such as from a linear accelerator, can control the placement of the dose very accurately by changing the intensity of the multiple rays of the treatment beam as a function of beam angle. Accurate computer control of the intensity of the radiation beam, over a range of angles, allows the irradiation of tumors having, for example, a concave cross-section--a dose pattern difficult to obtain with conventional radiotherapy techniques. Such a computer controlled system is described in U.S. Pat. No. 5,317,616 issued May 31, 1994 and assigned to the same assignees as the present invention and hereby incorporated by reference.
Despite the potential accuracy of x-ray or other photon radiotherapy systems, the ability to control the dose placement is limited by the physics of the photon beam and in particular the fact that the photon beam necessarily irradiates healthy tissue on both sides of an internal tumor as it passes through the patient. Further, the ability to minimize the total irradiation to heathy tissue with multiple beam angles in some cases is severely constrained by the presence of radiation sensitive organs near the tumor. The need to limit radiation to such organs may make some beam angles unavailable, increasing the dose to healthy tissue along the path of the other beam angles.
For this reason, it would be desirable to use protons instead of photons as the source of the radiation. By controlling the energy of the protons, the protons will stop at a precise location within the patient. In this way, tissue on the far side of the tumor with respect to the radiation source (the distal side) receives no radiation dose. Further, because the dose provided by a proton is concentrated at a "Bragg peak" around the area where the photons stop, the dose to healthy tissue on the near side of the tumor with respect to the radiation source (the proximal side) can also be reduced.
In present proton therapy systems, a beam of protons is collimated to the outline of the tumor and adjusted in energy to stop at the far edge of the tumor. Material is then inserted in the proton beam to reduce the energy of the protons and thus draw the point where the protons stop back through the tumor. As the wedge is inserted, the protons deposit an essentially even dose across the tumor. In practice, a rotating wheel with spokes of various thicknesses is inserted in the beam to deposit an even dose across the tumor.
This technique, which continuously exposes the patient to the beam of protons as the spokes are moved through the beam, dramatically increases the dose to healthy tissue on the near side of the tumor, obviating some of the benefits of proton radiotherapy.